1. Field of the Invention
The present invention relates to a liquid crystal display.
2. Description of the Related Art
Conventionally known is a liquid crystal display including a transmission type liquid crystal panel and a backlight unit arranged on the back side of the liquid crystal panel in parallel thereto for illuminating the liquid crystal panel from the back side thereof. The backlight unit has the same shape as that of the liquid crystal panel. The liquid crystal panel is configured by sealing a liquid crystal material between a pair of transparent electrode substrates opposed to each other through a seal, spacer, etc. A polarizing plate is bonded to each transparent electrode substrate of the liquid crystal panel.
FIGS. 1 and 2 show a general liquid crystal display in the prior art. As shown in FIGS. 1 and 2, light emitted from a backlight unit 2 is directed on a polarizing plate 4 having a first polarization axis. The polarizing plate 4 transmits only a part of the incident light oscillating in a direction parallel to the polarization axis of the polarizing plate 4, and absorbs the remaining part of the incident light. As a result, the transmitted light through the polarizing plate 4 becomes linearly polarized light. In the display state of maximum gray level, i.e., in the white display state shown in FIG. 1, the polarization plane of the above linearly polarized light is rotated 90xc2x0 by a liquid crystal layer 6 as shown by reference numeral 7 to enter a polarizing plate 8 located on the exit side of the liquid crystal layer 6. The polarizing plate 8 has a second polarization axis perpendicular to the polarization axis of the polarizing plate 4, so that the incident light on the polarizing plate 8 from the liquid crystal layer 6 is transmitted through the polarizing plate 8 to display a white image. Conversely, in the display state of zero gray level, i.e., in the black display state shown in FIG. 2, the polarization plane of the transmitted light through the polarizing plate 4 is not rotated by the liquid crystal layer 6, but the outgoing light from the polarizing plate 4 is transmitted through the liquid crystal layer 6 as it is. Since the polarization plane of the transmitted light through the liquid crystal layer 6 is perpendicular to the polarization axis of the polarizing plate 8, the transmitted light through the liquid crystal layer 6 is almost absorbed by the polarizing plate 8 to display a black image.
In a recent liquid crystal display, the luminance has become higher year after year, and the contrast has also been improved. With an increase in luminance, the amount of light 9 leaked from the polarizing plate 8 in the black display state as shown in FIG. 2 increases to result in an increase in luminance in the black display state, causing a problem in display quality. This phenomenon is referred to as dark defect. For example, a movie picture stored in a digital versatile disk (DVD) includes many dark images. If the dark images are displayed with brightness due to the above dark defect, the display quality of the movie picture is degraded and the movie picture is accordingly very hard to view. Such a problem is caused by the imbalance in performance between a light source (backlight unit) and polarizing plates.
The ability of the backlight unit continues to be improved year after year to achieve a high luminance. However, although the luminance of the backlight in the zero gray level has been improved, the light shielding ability of the polarizing plate has not yet been sufficient. That is, in the structure that a pair of polarizing plates having perpendicular polarization axes are located on the opposite sides of a liquid crystal layer, the leakage of light from the polarizing plate located on the exit side of the liquid crystal layer in the black display state cannot be completely prevented. Japanese Patent Laid-open No. 2000-180843 discloses a liquid crystal display including a plurality of polarizing plates arranged on the entrance side of a liquid crystal panel to improve heat resistance and light resistance. By arranging such a plurality of polarizing plates on the entrance side of the liquid crystal panel, most of unwanted light can be absorbed by the first polarizing plate (rearmost polarizing plate) to thereby reduce a thermal burden on the next polarizing plate, thus improving heat resistance and light resistance. However, this structure is considered to have little effect in preventing the leakage of light in the black display state to improve the contrast.
Japanese Patent Laid-open No. Hei 10-133196 discloses a liquid crystal display including a plurality of polarizing plates arranged on the exit side of a liquid crystal panel, wherein the polarizing plates have the same polarization axis. In this structure, the inside polarizing plate adjacent to the liquid crystal panel has a low polarization degree and the outside polarizing plate has a high polarization degree, thereby dispersing heat generated in the polarizing plates. This liquid crystal display is effective for a liquid crystal projector having a high heating value. However, a plurality of polarizing plates each having a high polarization degree are required for the improvement in contrast, so that the this liquid crystal display is not suitable for the improvement in total contrast, i.e., the improvement in display quality.
It is therefore an object of the present invention to provide a liquid crystal display which can be improved in contrast to thereby improve the display quality.
In accordance with an aspect of the present invention, there is provided a liquid crystal display comprising a liquid crystal layer having first and second surfaces opposite to each other; a first polarizing plate opposed to said first surface of said liquid crystal layer and having a first polarization axis; a second polarizing plate opposed to said second surface of said liquid crystal layer and having a second polarization axis perpendicular to said first polarization axis and a given polarization degree; and a third polarizing plate opposed to said second polarizing plate and having said second polarization axis and said given polarization degree.
Thus, the second and third polarizing plates having the same polarization axis and polarization degree are arranged on the exit side of the liquid crystal layer, so that the transmittance of the front (outside) polarizing plate in the black display state can be suppressed. Furthermore, a reduction in transmittance of the front polarizing plate in the display state other than the black display state can be minimized. As a result, the contrast of a display image can be improved, and the viewing angle can also be improved by the improvement in contrast.
Preferably, said third polarizing plate is bonded to said second polarizing plate to form a polarizing plate unit. The polarizing plate unit comprises a first adhesive layer opposed to said liquid crystal layer, a first protective layer formed on said first adhesive layer, a first polarizer formed on said first protective layer, a second protective layer formed on said first polarizer, a second adhesive layer formed on said second protective layer, a third protective layer formed on said second adhesive layer, a second polarizer formed on said third protective layer, a fourth protective layer formed on said second polarizer, and a protective film formed on said fourth protective layer. Preferably, the liquid crystal display further comprises a touch panel interposed between said second polarizing plate and said third polarizing plate; said touch panel being bonded to said second polarizing plate; said third polarizing plate being bonded to said touch panel.
In accordance with another aspect of the present invention, there is provided a liquid crystal display comprising a liquid crystal panel having a pair of transparent substrates and a liquid crystal layer sealed between said transparent substrates; a first polarizing plate opposed to one of said transparent substrates and having a first polarization axis; a second polarizing plate opposed to the other transparent substrate and having a second polarization axis perpendicular to said first polarization axis and a given polarization degree; and a third polarizing plate opposed to said second polarizing plate and having said second polarization axis and said given polarization degree.
Preferably, the liquid crystal display further comprises a touch panel interposed between said second polarizing plate and said third polarizing plate; said second polarizing plate being bonded to said liquid crystal panel; said touch panel being bonded to said second polarizing plate; said third polarizing plate being bonded to said touch panel.
In accordance with a further aspect of the present invention, there is provided electronic equipment having a liquid crystal display, said liquid crystal display comprising a liquid crystal layer having first and second surfaces opposite to each other; a first polarizing plate opposed to said first surface of said liquid crystal layer and having a first polarization axis; a second polarizing plate opposed to said second surface of said liquid crystal layer and having a second polarization axis perpendicular to said first polarization axis and a given polarization degree; and a third polarizing plate opposed to said second polarizing plate and having said second polarization axis and said given polarization degree.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.